1. Field of the Invention
The present invention relates to a color conversion apparatus which is applied to a system using a printer to print out a proof image representing colors of a color printed sheet before color printing is performed on a color printing press using a spot color in addition to process colors to convert image data for printing into image data for the printer, and to a color conversion program storage medium on which a color conversion program is stored which causes an information processing device such as a computer to function as the color conversion apparatus.
2. Description of the Related Art
Before printing a color image on a color printing press, a printer is used to print out a proof image having colors which look as much like those of the final image to be printed on the printing press as possible. In order to create such a proof image, a printing profile which describes the relation between image data and colors of an actual printed sheet and matches conditions such as the type of the printing press on which printing is performed and the conditions under which the printing press is used (the conditions required for a specific kind of printing, including the type of ink used, the quality of paper, and the type of the printing press, are herein called printing conditions) and a printer profile which describes the type of the printer on which a proof image is printed out and the conditions under which the printer is used (the conditions required for printing out a particular proof image, including the type of the printer, are herein called print conditions) should be known, image data for printing should be converted into image data for the printer according to the printing profile and the printer profile, and the proof image based on the converted printer image data should be printed out. In this way, the proof image having colors identical to the colors that will appear on the actual printed sheet can be provided. Usually, a printing profile that matches typical printing conditions is available from a printing company, and a printer profile is provided by the manufacturer of a printer designed for printing out proof images.
In order to convert image data for printing into image data for printer to output a proof image, typically a printing profile and a printer profile are combined into a link profile in LUT (Look-Up Table) form, the LUT is referred to, and interpolating operations are performed for lower bits for which no correspondences are described in the LUT.
The printing is typically performed by using process inks of four colors, CMYK (cyan, magenta, yellow, and black). In addition to the four process color inks, spot color inks can be used. In that case, it is not realistic to provide printing profiles for all possible combinations of the CMYK process inks and many spot color inks. Usually, printing profiles that suit printing in the CMYK process inks are provided. Therefore, the LUT created as described above is used for converting process color image data defined in a color space of the four colors CMYK for printing in the CMYK process inks into image data defined in a color space of CMYK for outputting a print image in color materials of CMYK, for example. For the sake of distinction, herein the CMYK for printing are referred to as C1M1Y1K1 and the CMYK for printer are referred to as C2M2Y2K2.
When printing in spot color inks in addition to the C1M1Y1K1 process inks in these circumstances, the problem is how to output a proof image for a printed sheet.
In this regard, Japanese Patent Laid-Open No. 2001-157074 proposes an approach. In the proposed approach, process color image data for printing (C1M1Y1K1 data) is converted into process color image data for printer (C2M2Y2K2 data or RGB data) with reference to a link profile as described above. For spot colors, a correspondence table describing the correspondence between the spot color names and the properties (such as the coordinates (L*a*b* values) in an L*a*b* color space) of the spot color ink with the spot color names, which are available from the manufacturer of the spot color inks, is created. The spot color name of a spot color ink used for printing is converted into L*a*b* values and then the L*a*b* values are converted into printer process color image data with reference to a printer profile. This process color image data is data corresponding to the spot color ink's solid color with a dot density of 100% (color with the maximum density). In addition to the process color image data corresponding to the solid color of the spot color, tone data (dot density % data) of the spot color used for the printing is also inputted. From these items of data, process color image data corresponding to the tone data of the spot color is obtained. Furthermore, the printer process color image data converted from the printing process color image data is combined with the spot color process color image data thus obtained to obtain image data to be inputted into the printer. With approach described in Japanese Patent Laid-Open No. 2001-157074, a fairly universal color conversion system can be readily built simply by obtaining properties (L*a*b* values) recommended by the manufacturer of spot color inks, in addition to a printing profile and a printer profile (and a link profile which is the combination of the two profiles).
The approach proposed in Japanese Patent Laid-Open No. 2001-157074 is simple, universal, and fairly good. However, there may be occasions where the appearance of the portion of an image printed out by a color conversion system built using the approach proposed in Japanese Patent Laid-Open No. 2001-157074 that corresponds to a spot color for printing is unsatisfactory and therefore fine adjustment of the color of the spot color on the printed image is desirable. This is because the appearance of the printing spot color on the printed image is slightly different from the predicted one due to various error factors: an error may be introduced when the spot color is converted into data with a given density because the properties (L*a*b* values) of the printing spot color obtained from the manufacturer of the spot color ink corresponds to a full density of 100%, or the printer profile may be imperfect and include an error, or an error may be introduced by interpolating operations which are necessary because the profiles typically are LUTs.
The approach proposed in Japanese Patent Laid-Open No. 2001-157074 provides a color conversion system which has a table that associates spot color names with L*a*b* values. When adjusting the color of the portion of a printed image that corresponds to a spot color for printing, the L*a*b* values associated with the spot color name are rewritten.
However, it is difficult to intuitively know which of the values of L*, a*, and b* should be rewritten to what extent in order to achieve almost the same color as a desired one. Accordingly, it is difficult to make adjustments.
Printers are available on which spot color inks can be used in addition to process inks. With the approach proposed in Japanese Patent Laid-Open No. 2001-157074, however, spot colors cannot be represented with spot color inks in combination with process color inks. Furthermore, the approach proposed in Japanese Patent Laid-Open No. 2001-157074 cannot separate a spot color of a binary printing image into other colors.